There is not yet a single unifying theory of cochlear amplification consistent with the organ of Corti cytoarchitecture, basilar membrane mechanics, and otoacoustic emissions (OAEs). Our central hypothesis is that the systematically organized Y-shaped structural elements between the reticular lamina and basilar membrane in the organ of Corti collectively form a mechanism for cochlear amplification in the best-frequency region of the basilar membrane, in which the angled outer hair cells (OHCs) provide an accumulating feed- forward force directed apically, and the oppositely angled phalangeal processes provide a feed-backward force directed basally. The feed-forward and feed-backward (FF/FB) amplification theory will be tested using anatomically realistic fluid-coupled 3D finite element computational models for the mouse and gerbil organs of Corti, constructed from two-photon and confocal microscopy images. After validation against previous physiological measurements and modeling results, the new models will be used to test the effects of FF/FB forces on cochlear amplification, as well as the effects of tectorial and basilar membrane mechanics. The hypothesis that the FF/FB amplifier concepts are compatible with theories and measurements of stimulus- frequency OAEs and distortion-product OAEs, and that selective modifications to the structure of the organ of Corti will produce predictable results, will be tested both in the model and experimentally using wild-type mice and alpha-tectorin protein mutated mice (TectaC1509G/+) that feature a shortened tectorial membrane. Cochlear models have typically assumed that the OHC force output is proportional to the stereociliary force input, with a gain ? assumed to be independent of cochlear location and frequency. We will improve upon this by creating a model for the OHC receptor potential that accounts for the basolateral conductances and cell wall capacitance, which we will then combine with an anatomically and physiologically realistic model for somatic motility in order to determine realistic values for ? as a function of location and frequency. The resulting ?(x,r,f) model will then be integrated into our FF/FB modeling frameworks as a further test of our central hypothesis. The scientific contributions stemming directly from this research are expected to be 1) a detailed 3D description of the Y- shaped elements in the organ of Corti across the different OHC rows, from base to apex; 2) an incorporation of this information into computational models for testing the FF/FB amplifier theories with realistic anatomy; 3) an improved understanding of how mechanisms of OAE generation and propagation relate to the FF/FB amplification theory; and 4) an enhanced understanding of the contributions of OHC basolateral conductances, receptor potential, and somatic motility to cochlear amplification. The resulting models will provide powerful new tools for future cochlear mechanics studies, including those involving normal, genetically modified, and regenerated mouse cochleae, and, due to homologs between humans and mice, the human cochlea as well.